It is known in the art to refrigerate natural gas streams to facilitate the separation of impurities therefrom, to facilitate the separation of various components of the gas and to liquify the gas for storage and shipment. For example, it is common practice to refrigerate a natural gas stream to a temperature low enough to condense hydrocarbons heavier than methane. The liquified heavier hydrocarbons are then readily separated from the gas predominating in methane. One problem in reducing the temperature of a natural gas stream below 32.degree. F. is the condensation of water vapor usually contained therein, with the resultant freezing thereof to form ice crystals and/or water-hydrocarbon hydrate crystals. In fact, water-hydrocarbon hydrate crystals are usually formed at temperatures above 32.degree. F. sometimes as high as 40.degree. F. Such ice crystals and hydrates tend to plug the heat exchangers in the chilling or cooling stage and make operation thereof impossible for a short time. Carbon dioxide and hydrogen sulfide, if present in the gas, will also freeze out of the gas if it is cooled to a temperature in the order of about -115.degree. F.
In order to overcome the above-mentioned formation of ice crystals and solid hydrates in natural gas processing systems, the water and carbon dioxide are removed to the extent possible before the natural gas feedstock is cooled below the temperature at which such solids will form. One method to accomplish this comprises contacting the gas with a high boiling (low vapor pressure) liquid dessicant, such as diethylene glycol or triethylene glycol, to absorb the water. High boiling dessicants are used in such processes so as to minimize introduction of the dessicant into the gas stream by evaporation. Such processes have the disadvantage of high operating costs, particularly in the recovery of the dessicant. In addition, glycols become very viscous at low temperatures and present a handling problem in and of themselves. Another method which has been employed is to pass the gas through towers containing solid adsorbents, such as silica gel, molecular sieve, solid caustic, etc. Such adsorbents and the operating costs associated therewith are expensive. Because of the expense of such operations, cheaper solutions to the problems have been sought. More recently, an antifreeze such as methyl alcohol has been added to the natural gas feedstock and passed together with the feedstock through low temperature heat exchange units. As the water is condensed from the gas, it is absorbed by the alcohol to form a liquid alcohol-water phase which separates from the gaseous feedstock being cooled. While, in general, this process has been more economical than the use of adsorbing dessicants, the process is not without problems. For example, when heavier hydrocarbons condense from the natural gas in contact with the alcohol-water phase, two liquid phases are present in the equipment, a hydrocarbon phase and a water-alcohol phase. Since the alcohol is also soluble in the hydrocarbon phase, it may transfer from the water phase to the larger volume hydrocarbon phase leaving insufficient alcohol in the water phase which will then freeze and plug the equipment.
In order to alleviate the above-mentioned problems, it has been common practice to contact the natural gas with an aqueous liquid dessicant-antifreeze agent in a manner to remove at least a portion of the moisture contained in the gas and introduce a sufficient amount of the dessicant-antifreeze in the vapor phase into the gas so as to prevent formation of solids in subsequent low temperature treating steps. Further improvement is obtained by separating the aqueous, dessicant-antifreeze from the gas and other hydrocarbons being treated and then fractionate the water and dessicant-antifreeze mixture to separate water therefrom and thus adjust the water content of the aqueous solution utilized to treat the gas for recycle to the gas contacting step.
While the above-mentioned recovery of water and dessicant-antifreeze for reuse is effective where the dessicant-antifreeze contains only water, serious problems exist when the water and dessicant-antifreeze contains other impurities. For example, hydrogen sulfide, if present in significant amounts in the natural gas being treated, will concentrate in the dessicant-antifreeze phase in the fractionator. Because of this, the dessicant-antifreeze is unacceptable for reuse and must be disposed of. While heat stripping of the hydrogen sulfide from the dessicant-antifreeze can be utilized to purify the dessicant-antifreeze, such an operation involves substantial investments operating costs and results in significant losses of the dessicant-antifreeze. In addition, such heat stripping produces a gaseous phase containing the hydrogen sulfide which is normally vented to the atmosphere and, of course, causes undesirable pollution problems.